Ribbon tension control



y 6, 1969 .I- w. SPEARS v 3,442,366

RIBBON TENSION CONTROL Filed March 18. 1966 FIG. 2

INVENTOR.

JESSE W. SPEARS AITORNEY.

United States Patent US. Cl. 197-451 4 'Claims ABSTRACT OF THE DISCLOSURE Ribbon tension is controlled in an incrementing ribbon feed system by a pawl and ratchet brake device actuated by direct ribbon tension sensing to selectively positively hold or completely release the supply spool and thereby maintain ribbon tension within predetermined limits.

Disclosure of the invention This mechanism accomplishes accurate ribbon tension control through the use of an ultra simple structure. In fact, this mechanism is so simple and its effectiveness so great that it is possible to employ substantially longer ribbon length without impairing print quality or feed reliability due to length related tension changes, while also reducing the overall cost of the ribbon feed mechanism.

High quality typewritten material is produced through the use of so-called total release or single pass ribbons. It is desirable, from economic and user acceptance considerations, to minimize the frequency that a new single pass ribbon must be inserted. To this end, large reels and thin ribbon have been employed whenever possible. In traversing-printhead printers, such as the IBM Selectric typewriter where the ribbon and its feed mechanisms are carried bodily along a stationary printing line, large diameter reels or spools become impractical. A low ribbon change frequency is obtained by use of a precision ribbon lift mechanism that scatters strike points over a wide ribbon in a vertically overlapped pattern.

As any web material is delivered between a pair of spools, the tension therein will tend to vary due to geometrical and inertial changes. In a typewriter ribbon system, a certain amount of tension variation does not adversely affect ribbon feed or print quality. However, an excessively low ribbon tension can permit ribbon to become untracked from its feeding device with complete printing failure. An excessively high ribbon tension can cause the ribbon to wrinkle with resultant mangled characters or even cause the ribbon to break. Variation in ribbon tension also adversely affects proper operation of a tension sensitive device such as the precision ribbon lift mechanism mentioned above.

Currently, ribbon tension variation has been limited by limiting the ribbon spool size, such that dimensional variations between full and empty spools is limited. This solution is less than totally satisfactory since it defeats the objective of minimizing the ribbon change frequency. Another partial solution has been to provide a simple spring loaded brake acting directly on the ribbon periphery to create a drag that varies inversely with the geometrical changes in the ribbon. This method does not take into account the inertial effects of geometrical changes and thus is also less than totally satisfactory.

Accordingly, it has been an object of my invention to devise and develop a mechanism for controlling the tension in a continuous web fed from a spool to eliminate limitations on spool size heretofore caused by web tension variations.

Another object of my invention has been to devise and develop a simply constructed web tension control mecha- 3,442,366 Patented May 6, 1969 nism especially for printer ink ribbon feeding to make accurate control economically practical.

My tension control mechanism is principled upon a novel, simple mechanical closed loop concept for controlling a step-function-operative brake, and thereby maintain ribbon tension within predetermined limits.

Ribbon tension at any instant is a function of both applied forces and reactive or inertial forces. For a given ribbon spool geometry, variations in inertial forces are fixed. Applied forces can be varied and by appropriate variation, can compensate for the variations of the inertial forces. By this invention I provide a ratchet and pawl combination which, when mutually engaged, totally prevents rotation of a supply spool. Conversely, when the pawl and ratchet are not mutually engaged, they exercise no control whatsoever over the supply spool. In other words, the pawl and ratchet constitute a step-functionoperative control which is either on or off. The pawl is spring-biased to normally prevent motion of the supply spool. When ribbon tension exceeds a predetermined maximum level, the pawl is driven from this normal position. The pawl is permitted to re-engage after suflicient ribbon has been fed to permit the ribbon tension to fall below this maximum level. During the time interval that the pawl is disengaged, the ribbon tension will be controlled essentially by the inertial or reactive forces acting on the ribbon spools. If the reactive forces are low, as when the angular acceleration of a supply spool is low during early stages of feeding a ribbon, the ribbon tension will quickly fall below the prescribed maximum and the pawl will re-engage immediately. Continued motion induced by the feed mechanism quickly restores a minimum tension as ribbon is fed while the supply spool is held by the pawl. At the other extreme where the active forces are high due to high acceleration of the supply spool, the pawl, when removed from the ratchet, will remain out of the ratchet until acceleration has lowered, and the ribbon tension falls below the prescribed maximum. It can be seen that the mechanism will maintain ribbon tension substantially constant within limits while permitting only transient deviations therefrom which do not adversely affect the ribbon feed. Of particular importance is the absence of any cumulative ribbon tension between feed cycles.

Another phase of my invention involves the employment of a spring connection between the ribbon tension sensing or engaging mechanism thereof and the pawl, such that all changes in ribbon tension are not immediately delivered to the pawl. A time lag is thus provided and the sensitivity of the mechanism is reduced to a practical level, considering normal control problems of hunting or chattering. This phase is practically implemented by employing a unitary spring wire as both the sensing mechanism and the pawl.

These and other objects, features, and advantages of my invention will be more apparent to those skilled in the art upon reading the following description of a preferred embodiment of my invention wherein specific reference is made to the accompanying drawing, of which:

FIGURE 1 is a partial perspective view of a ribbon supply mechanism constructed in accordance with my invention; and

FIGURE 2 is an enlarged top or plan view of the ribbon supply mechanism, more clearly showing the tension control mechanism therefor.

My invention is shown as applied to a ribbon feed mechanism generally as described in the article entitled Typewriter Ribbon Feed, by J. B. ODaniel and L. E. Palmer, published September 1963 in the IBM Technical Disclosure Bulletin, vol. 6, No. 4, pp. 1718. The feed mechanism is part of a typewriter or printer like that disclosed in US. Patent 2,919,002 entitled Selection Mechanism for a Single Printing Element Typewriter, inventor L. E. Palmer, issued Dec. 29, 1959, having a platen or reaction part 11 and a printhead 12 defining a printing station 13 therebetween. The printhead 12 is supported on a carrier 14 that is movable laterally to traverse a writing line 15. The ribbon feed mechanism is also mounted on the carrier 14 and includes a ribbon or web supply spool 21, a takeup spool or used ribbon collection means 22 (FIGURE 2), ribbon lift guides 23' and incrementally operating ribbon feed capstan and pinchroll mechanism or meshing rubber gears 24.

In operation, the capstan mechanism 24 is driven by a stepping drive (not shown) to move a constant linear amount of a web or ink ribbon 26 past the printing station 13. The guides 23 are thrust upward by different amounts each print cycle by a cam device (not shown) to scatter strike points in a vertically overlapped pattern over the ribbon. The takeup spool 22 is supported for rotation by a shaft 27 that engages a spool hub 22a and is driven by garter spring 25 to collect the ribbon pulled by the capstan mechanism 24.

The supply spool 21 is supported on a ratchet wheel 31 and is keyed thereto by mechanism 32. A pin shaft 33 rotatably supports the ratchet wheel 31 and spool 21. My control mechanism includes a pawl or detent 34 that is dimensioned to fit in and tightly engage the teeth of the ratchet 31 and provide an on-off or step-function operated brake in cooperation therewith. The pawl 34 is pivoted for movement about an axis 35 and is biased counterclockwise by a spring 36 into a normal condition of engagement with teeth of the ratchet 31. A tension sensing finger 37 engages ribbon 26 coming from the supply spool 21 and is movable to the right in response to a tension increase. The sensing finger 37 is connected to the pawl 34 through a long flexible arm or spring portion 38 to permit a degree of finger movement without a total corresponding movement of the pawl 34. This internal spring action reduces the sensitivity of the control to a practical level and thereby avoids hunting or chattering. Conveniently, the pawl 34, the arm 38, and the finger 37, are all made of an integral or unitary piece of bent, spring, so-called piano wire or resilient steel. This piano wire is reinforced at the pivot axis 35 by a small block 39.

Operation In operation, ribbon from a full supply spool 21 is threaded about the sensing finger 37 of the control mechanism, through the ribbon lift guide 23, past ribbon al- 'ready wound on takeup spool 22, through the capstan mechanism 24 and is attached to the takeup spool 22. During the early print cycles the supply spool 21 is required to rotate only very small amounts to deliver the required length of ribbon past the printing point 13. Accordingly, a very small acceleration is required of the supply spool 21 and inertial forces will be low. The ribbon 26 pulled past the printing point 13 first moves the sensing finger 37 by merely bending the flexible arm 38. Ribbon tension builds up, due to the pawl 34 which prevents ribbon feeding by the supply spool 21. When ribbon tension has exceeded a predetermined maximum level, as controlled by the spring 36, pawl 34 is lifted from the ratchet 31 to the broken line position in FIGURE 2 and the spool 21 is freed to accelerate. Ribbon tension will immediately drop due to the low rate of acceleration required of the large diameter full spool 21 and the pawl 34 will be reinserted in the next adjacent tooth by spring 36.

As ribbon is continued to be fed and the diameter of supply spool 21 is reduced, greater angular accelerations are required to feed the measured amount of ribbon from the supply spool 21. The supply spool thus poses a greater drag, and hence an increased ribbon tension during the period that the pawl 34 is removed from the ratchet 31. However, along with acceleration the ribbon tension falls near the end of the feed stroke and immediately the pawl 34 is restored by the spring 36 and arrests motion of the ratchet 31. Reinsertion of the pawl 34 prevents over-run by the spool 21 and thus maintains the tension at a desirably high level.

It will be seen that my invention provides an effective, and yet simple mechanism for maintaining a substantially constant ribbon tension over a wide variation in inertial tension creating conditions.

It will be understood, however, that variations may be made in my embodiment without departing from the spirit and scope thereof. It will be particularly appreciated that the braking forces herein employed to brake the ribbon spool are in no way proportionally related to ribbon tension, although these forces are controlled by the ribbon tension itself. Accordingly, there is no limiting relationship between the desired tension level and the braking force required, as would be the case of a direct rubbing type friction brake.

Having thus described my invention, and a specific, preferred embodiment thereof to illustrate its principles, I claim:

1. In web feeding apparatus including means for rotatably supporting a supply spool, and incremental operating means for pulling a continuous web from the supply spool; the improved web tension control mechanism comprising:

a ratchet rotatable with said supply spool,

a detent mounted adjacent said ratchet and movable into engagement therewith for preventing rotation of said supply spool,

spring means biasing said detent toward engagement with said ratchet,

a movable tension sensing part engageable with the web pulled from the supply spool, and

means including a spring for yieldably interconnecting said tension sensing part with said detent in opposition to said spring means; whereby increasing tension in the web resulting from operation of said incremental operation means will effect first a yielding of said yieldable interconnecting means and thereafter movement of said detent against the opposition of said spring means to disengage said detent from said ratchet when the web tension exceeds a predetermined level.

2. Web feeding apparatus as defined in claim 1 wherein said detent, said interconnecting means, and said tension sensing means are formed as a single integral part.

3. In a printer having a platen, a laterally movable carrier cooperable with said platen for traversing a writing line, printhead means supported on said carrier for impacting with said platen to print characters, and ink ribbon feeding means supported on said carrier for delivering ink ribbon past said printhead means in incremental steps, wherein said feeding means includes means for rotatably supporting a supply spool, means for guiding said ribbon between said printhead means and said platen, and means for incrementally moving said ribbon past said printhead; the improved ribbon tension control mechanism comprising:

a ratchet rotatable with said supply spool,

a detent mounted adjacent said ratchet and movable into engagement therewith for preventing rotation of said supply spool,

spring means biasing said detent toward engagement with said ratchet,

a movable tension sensing part engageable with the ribbon pulled from the supply spool, and

means including a spring for yieldably interconnecting said tension sensing part with said detent in opposition to said spring means; whereby increasing tension in the ribbon resulting from operation of said incremental ribbon moving means will efiect first a yielding of said yieldablc interconnecting means 5 6 and thereafter movement of said detent against the 3,171,530 3/1965 ODaniel et al. 197-151 opposition of said spring means to disengage said 3,283,876 11/1966 Kern 197-151 X detent from said ratchet when the ribbon tension exceeds a predetermined level. FO G A S 4. A printer as defined in claim 1 wherein said detent, said interconnecting means, and said tension sensing 5 252778 11/1948 Swltzerland means are formed as a single integral part. OTHER REFERENCES References Cit d IBM Technical Disclosure Bulletin, Typewriter Rib- UNITED STATES PATENTS bon Feed, ODaniel et al., vol. 6, No. 4, September 1963, pp. 17 and 18. 1,399,480 12/1921 [Helmond 197151 1,947,0 4 2/1934 Butler 1 7 2 X ERNEST T. WRIGHT, JR.,Prima)y Examiner. 2,107,074 2/1938 Hineline. 2,906,203 9/1959 Grosse 101-336 US. Cl. X.R. 3,069,107 12/1962 Hirt 242 7s.43 X 15 242 7s.43

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,442,366 Dated Mav 6. 1969 Inventor(s) J. W. Spears It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 5; Delete "1" and insert -3--.

SIGNED AN'D SEALED NDV 181969 (SEAL) Attest:

Edward M. Fletcher, Jr.

WILLIAM E. SOHUYLER, JR.

estin Officer A g Commissioner of Patents 

